Here’s something you already know: The Cubs starting rotation has been really, really good so far this year. Even with Jake Arrieta pitching more like a very good pitcher than a super-human out machine and John Lackey landing himself on the disabled list today, this is a rotation that’s incredibly strong, front to back.

Lackey’s injury isn’t of much concern to me, as there’s no reason he shouldn’t be back as soon as his 15-day DL stint (which can be made retroactive to Aug. 14, meaning he can be back as early as Aug. 29/30) is completed. And, really, for a 37-year-old with nearly 14 years of big league wear and tear on his arm, the extra rest provided by his DL stint may be a blessing in disguise.

So, assuming the Cubs starting five on the final day of the season is still comprised of Arrieta, Jon Lester, Kyle Hendricks, Lackey and Jason Hammel, a big question looms: Who gets left off of the playoff rotation?

If you asked me that question heading into the season, the answer would’ve been easy – the obvious answer would’ve been Hammel. However, looking at where things stand 120 games into the season, that’s not such an easy answer anymore.

It’s pretty obvious that Arrieta, Lester and Hendricks, who still owns the second best ERA in baseball at 2.19, trailing only a guy who hasn’t pitched since June 26 (Clayton Kershaw – 1.79 ERA*), will be on the playoff rotation.

*(Actually, as a point of order, Hendricks is a few Kershaw-less days away from taking the league lead. Pitchers need to have 1 IP for every game played by his team to qualify for the leaderboard. Right now, Kershaw has 121 IP, and the Dodgers have played 120 games. So, after Saturday’s game, he’ll be off the leaderboard.)

That brings us to Lackey and Hammel. Statistically, Hammel has an obvious edge. Currently sporting a 2.75 ERA/1.065 WHIP and a career-low of 6.8 H/9, Hammel seems to be getting stronger as the year goes on. Since the All-Star break, he’s a perfect 6-0 with a 0.95 ERA (!!!) and a 0.895 WHIP. Further, if you take away one (really) bad outing on July 1 in which he gave up 10 ER in only four innings, his ERA would be at 2.14, which would replace Hendricks as the second-best ERA in the league.

Meanwhile, Lackey’s numbers currently sit at 3.41 ERA/1.042 ERA, though he’s also sporting a career low 6.9 H/9, and his FIP is actually lower at 3.72 (Hammel’s is 4.09). Still, Hammel’s absurd numbers, especially in the second half, are hard to overlook.

Of course, as much as it pains me to say it as a stat guy at heart, numbers aren’t the only consideration here. Also key to this decision is the fact that, essentially, the sole reason Lackey was signed was for the playoffs. In total, he’s played in 15 different playoff series and made 23 total appearances in his playoff career – 20 of them as a starter – and he’s pitched very well.

In a combined 127.1 playoff innings, Lackey has posted a 3.11 ERA/1.217 WHIP. That’s why the Cubs signed him – that’s what he’s here for. So how do you take that and put it in the bullpen during the playoffs? I don’t think you can.

But, similarly, how do you move Hammel to the pen after what he’s done? Who takes a guy with a 2-point ERA over the course of a season and move him to the bullpen for the playoffs? Really, Joe Maddon is left with three options. Move Hammel to the pen – which seems probable, move Lackey to the pen – which seems highly unlikely, or do something entirely Maddon-like.

That is to say: Have your 1-3 starters pitch as per usual, and then plan some funky situation for Game No. 4 where two starters split 4 innings a piece. It sounds like something Maddon would try; but it also sounds like something the at-times-meat-headed Lackey would loathe the thought of.

So, perhaps the solution is for Hammel and Hendricks to try this split. Of course, then you’re limiting the output of your best (in terms of results this season) pitcher.